Field
This invention relates to handling interference in power and communications systems. More particularly, the invention relates to methods and systems for suppressing interference in communication signals and power signals transmitted over electrical power lines.
Description of the Related Art
Electrical power lines are sometimes used to carry signals and data in addition to electrical power. A power line communication (PLC) system, for example, may allow end-user devices and utilities to transmit data and enable devices to respond to commands. One application for power line communication is in a “smart grid”. The term “smart grid” may be used to refer to a control system that integrates digital computing and communication technologies and services with the power-delivery infrastructure, such as contemplated in the Energy Independence and Security Act of 2007.
Power line communications, such as those used in smart grid applications, may not function properly if reliable communications are not available among users (for example, among grid operators, utilities, end users, and electrical devices). Utilities increasingly require improved communications methods & technologies to collect data and computing power and software to visualize and control grid operations in real time.
One problem that may be encountered in PLC systems is interference. A low-frequency power line channel tends to be a harsh, complex, reactive environment for any form of communications. The power signal itself (the 60 Hz “fundamental”) may produce a form of distortion that is problematic for secondary signals (such as communication signals) which are introduced into the channel. For example, the reactive nature of the channel, combined with the relative heft of the primary stimulus (the fundamental, or the power signal itself), may produce a form of channel-initiated modulation which may have a detrimental effect on such secondary communication signals. This channel-initiated modulation may express itself in the combined channel (containing the power signal, the communication signal, and other noise) as a form of noise which is time-synchronous with (or “coherent with”) the power signal and which affects all signals in the channel. This time-synchronous or coherent noise may also be referred to as “images” or “ghosts” or “side lobes”. The coherent noise may be perceived as a recursive replication of the original communication signal, a recursive replication of some component of the original communication signal, or periodically recurring energy which distorts the communication signal. Image signals may be particularly problematic for communications scenarios which rely on multiple closely spaced, narrowband subcarriers (for example, narrowband frequency division modulation or similar approaches).
Transmission of data via an active power line is a difficult task. Among other things, the pre-existing power signal may cause a number of problems in the system. These problems may include pseudo-stationary interference from harmonics of the fundamental, other forms of synchronous noise, or a form of “blowback” into the data transmitter from the fundamental and harmonics, among others. The structure of the power line network may also interact with low frequency secondary signals that are introduced to the network for the purposes of communication. This interaction may appear as a form of amplitude modulation of the communication signal. The amplitude modulation envelope may be time-synchronous or coherent with the fundamental. The distorting amplitude modulation envelope may be imposed on any/all secondary signals in the channel. This type of channel-induced distortion may be problematic for data communications because amplitude modulation tends to create a proliferation of harmonically-related images of the communication signal or components of the communication signal. As image signals proliferate in the channel, the availability of idle or clear spectrum for additional subcarriers may be drastically reduced, and thus the placement of those subcarriers may become difficult.